The present invention relates to a polycrystalline silicon thin film to be used as a semiconductor, such as a thin film transistor or a solar cell, and in particular to a polycrystalline silicon thin film formed on a substrate other than a single crystalline silicon substrate, e.g. a transparent substrate such as a glass substrate, and also to a polycrystalline silicon thin film transistor using such a thin film.
Polycrystalline silicon thin film is an aggregation of a large number of silicon crystals of sizes of several hundreds of .ANG. to several tens of .mu.m. Compared with amorphous silicon, the mobility of electrons is higher by one to two orders of magnitude and has the excellent property that it can be formed on any substrate, except crystalline silicon, such as alumina or graphite, as has not been achievable with single crystalline silicon. A transistor using a polycrystalline silicon thin film has higher mobility than an amorphous silicon transistor and even peripheral driving circuits can be incorporated on the same substrate.
In general, such polycrystalline silicon thin film is formed by a thermal CVD method on a substrate at a high temperature of 600.degree.-700.degree. C. or more, and therefore the substrate must be resistant to such a high temperature. Also, because the substrate, on which the thin film and transistor are formed, can be lighted by back-light if it is transparent, it is suitable for use as a liquid crystal display, and there are strong demands to use transparent substrates, e.g. a glass substrate, as the substrate for polycrystalline silicon. However, the strain point of normal glass is 600.degree. C. or lower, and it is impossible to form polycrystalline silicon on it. Therefore, when polycrystalline silicon thin film is formed on a glass substrate, heat-resistant glass, such as quartz glass, must be used. This is not suitable for mass production because it is very expensive.
Also, polycrystalline silicon thin film has a higher percentage of orientation (220) at low temperature, and the percentage of (220) orientation is decreased at higher temperature, while the percentage of (100) orientation tends to increase with the temperature. Because conventional type polycrystalline silicon thin film is formed by a thermal CVD method, the process temperature is high. As a result, the percentage of (220) orientation is low, and the percentage of (100) orientation is increased, whereas the usable substrate is limited because process temperature is high. In this respect, there has been a strong demand for a new thin film forming technique, which provides a lower percentage of (220) orientation and a higher percentage of (100) orientation in a low temperature process.
It is said that, when the percentage of (220) orientation is higher, transistor characteristics, particularly effective carrier mobility, is increased and the change over time during continuous operation is decreased. Thus, a proposal has been made to increase the percentage of (220) orientation. While this is on imperfect polycrystalline silicon with relatively smaller grain size, it is not certain whether it can be applied to polycrystalline silicon with larger grain size. On the contrary, when the percentage of (220) orientation is decreased. This results in a lack of surface flatness and a lower yield in microstructure fabrication.
Further, the results of a recent study reveal that, in the case of a polycrystalline silicon formed by the thermal CVD method, there are many voids between silicon grains because the temperature during manufacture is high, and that the dangling bonds at grain boundaries deteriorate electrical properties. To correct such inconveniences, it is necessary to passivate the grain boundary with hydrogen, and this is very disadvantageous.
Because glass substrates with a low strain point are relatively cheap, they are useful for the manufacture of thin film transistor. However, silicon thin film obtained on such a glass substrate is amorphous because of the restriction on the process temperature. Thus, a technique to form polycrystalline silicon thin film, with excellent electrical property, on glass substrate with a low strain point has not yet been established. When a polycrystalline silicon thin film is formed at relatively low temperature by a conventional technique, the silicon at the interface with the substrate is not crystallized (amorphous silicon) or crystals are found in finer state (microcrystalline silicon), and it is not possible to obtain polycrystalline silicon with large grain size.
In this respect, attention is focused now on a plasma CVD method to produce a polycrystalline silicon thin film at a low temperature (Japanese Provisional Patent Publications No. 63-157872 and No. 63-175417). In these methods, a large quantity of hydrogen gas is used as one of the components of reaction gas, and polycrystalline silicon thus obtained contains hydrogen between silicon grains compared with the product obtained by the thermal CVD method. Thus, it is not necessary to passivate polycrystalline silicon with hydrogen after the manufacture.
However, polycrystalline silicon thin film obtained by the above method contains hydrogen in a proportion of about 2.5 atoms % or more, and the grain size of the silicon is about 500 .ANG. at the most.